hi, im running out of pins on my 328 id like to add a 4 button keypad to it ive seen somewhere you can add extra switches / pins by using an array of different value resistors not sure how it works maby by different voltages to a pin somehow?

What else do you have connected to your Arduino? It's often possible to use the same pin to drive multiple devices. For example, if you already have an LCD display connected, you can probably share 4 pins between the LCD and the keypad.

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

2) Put other devices on shared buses, such as I2C and SPI, to free up other pins. This can require use of port expanders such as the PCF8574 which gives 8 input/outputs for 2 shared pins on the uC.

3) Use a voltage divided to read multiple buttons on an Analog pin. It needs tolerances building in and debouncing.

4) Matrix buttons and make use of the Keypad library to give you up to 9 buttons on 6 pins, 16 on 4 pins, 25 on 5 pins etc. Upto 4 x 4 (8 pins) can also be used together with a PCF8574 and Keypad_I2C to get 16 buttons on 2 pins. This can be repeated on the same two I2C pins to have 32, 64, 128 buttons or more.

5) Use cheap shift registers, such as the 8 bit 74HC595, and cascade to expand input/outputs using just 3 uC pins. limited only by the processing overhead to drive the number of shift registers you incorporate.